In recent years, multi-band and broadband use is progressing for want of frequency bands in the field of mobile phones, and therefore, power amplifiers used in the mobile phones are demanded to have increased power added efficiency over a broadband range, which is an essential characteristic.
The power added efficiency of a power amplifier depends dominantly and largely on the characteristics of an output matching circuit of the power amplifier. A semiconductor amplifier at the final stage of the power amplifier performs nonlinear operation in high power output, and therefore, circuits for processing harmonic signals generated as nonlinear components are the key. Particularly, a circuit for processing a second-order harmonic as a main harmonic is the key. In view of this, in general, a resonant circuit, in which a resonance frequency is matched to the frequency of a second-order harmonic, is connected to an output circuit, and a target harmonic is short-circuited or open-circuited to suppress excessive increase in output level of the second-order harmonic. The resonant circuit suppresses only the second-order harmonic in the entirety of an amplified output signal and is called, therefore, a second-order harmonic trap circuit in general. Further, the second-order harmonic is short-circuited generally in such a manner that a trap line or an inductor component having an inductance of 1 nH or larger is connected in series to a capacitor to form a resonant circuit having a second-order harmonic frequency as a resonance frequency and the resonant circuit is shunt-connected to the output circuit.
Patent Document 1: Japanese Patent Application Laid Open Publication No. 2002-43873
Patent Document 2: Japanese Patent Application Laid Open Publication No. 2000-40928